This invention relates to a vacuum pump with a back-up bearing assembly and, more particularly, to such a pump with an assembly provided with a thermal compensator and an axial position sensor.
A vacuum pump, such as a turbomolecular pump, typically having a rotor assembly with integral rotor disks and a pump body with integral stator rings is described in the U.S. Pat. No. 5,238,362 that is incorporated by reference herein. Such a turbomolecular pump rotor, when supported by magnetic bearings, may preferably be provided with back-up bearings which will allow the rotor to spin down safely without damaging the pump, when for example the primary magnetic bearing levitation system is disabled due to a loss of electric power, an overloading of the magnetic bearings or a malfunction of the control electronics takes place. It is further preferable to place such back-up bearing assembly inside the drive shaft of the rotor assembly to provide a compact design and to allow for ease of maintenance as it is disclosed in the now U.S. Pat. No. 5,883,374 filed by Varian Associates, Inc.
In addition, the heat generated by the vacuum pump can be transferred to the drive shaft of the rotor assembly causing thermal expansion. Such growth in the drive shaft can alter the clearances between the back-up bearing surfaces and the inner surfaces of the drive shaft, thus affecting the engagement of the back-up bearings with the drive shaft. The back-up bearing assembly must be able to compensate for possible changes in the clearances to provide for optimum performance, as well as provide for means of monitoring such changes.